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odor sensitivity - mercapatan?
Saturday, October 3rd, 2015 03:46 amI mentioned a house before, which had a natural gas leak. The smell was so offensive and sickening to me that I could not stand to remain in the house for more than a few minutes. Yet 3 other people, including the 2 that lived there, smelled nothing at all, and the 4th person(my mom) only smelled it slightly. The house had a gas stove which had been used recently to cook a meal, so it was suggested that I was simply smelling the pilot light, or perhaps some of the gas left over from the cooking.
Now, I've been in the house again several times, and though the stove hadn't been used in a long time, I could still smell gas. It wasn't nearly as strong as before, but still quite offensive. The smell hit me opening the outer door of the house already, before even opening the main door. The smell was strongest in the lower kitchen cabinets under the stove and under the sink - enough to make me gag. But again, no one else who visited the house smelled anything unusual, even in those same cabinets.
I strongly suggested the owner get the pipes and stove checked for a gas leak. But procrastination ensued.
Finally I procured a combustible gas detector, as by now I was now very curious whether it was really gas that I was smelling, and how much of it there was, that I could smell it so strongly, yet no one else could.
In the cabinet under the stove, the detector showed levels of up to 6000ppm of natural gas, or 11% of the LEL (lower explosive limit). Inside the rest of the house, where the odor was still fairly strong to me, it showed levels of about 50ppm.
The owner finally did get it checked out. The gas company rep confirmed a "small" leak (not with his nose; with his own meter), and shut off the gas. There was a shut-off valve right in the cabinet which I hadn't even noticed due to being so repulsed by the smell.
The house was aired out. Now, with the main gas smell gone, I noticed that the house smelled rather musty - something I hadn't been able to tell before, due to the stronger smell of gas. But I still kept noticing some whiffs of gas too. So I checked the cabinets again, and found another leak, this time by the pipes under the oven. There, the detector showed levels of only up to 300ppm, but it was still strong enough to make me gag.
If the gas rep considered levels of 6000ppm a small leak, I suppose that levels of 300ppm (~0.5% LEL) would be considered inconsequential. And the 50ppm or so (~0.1%), which I could also easily detect, would be considered even more inconsequential.
Anyway, I found a pamphlet which says:
"Both propane and natural gas are colorless and odorless. For safety reasons, an odorant (Mercapatan) is added so that any leak can be detected by smell. The common detection threshold for smelling the gases is around 20% of the lower explosion limit (LEL)."
So apparently most people can smell it at 20%, but I can smell it at 0.1%. I'm really curious if this is genetic, and how many other people there are who are similarly sensitive to the smell of mercapatan.
Also, propane smells different to me than natural gas does. I wonder if they are they truly odorless to everyone, and is it only the additives that I smell? But it may be that the smell I associate with propane is the smell of the smoke when it is burned, more than the gas itself.
I also wonder now, how many smells which are so strongly offensive to me, other people don't even smell at all. (As opposed to them smelling it but not being bothered by it, as I had generally assumed.) I've been thinking about this a lot lately in light of the above, and might do another post on it.
Oh, another note. According to several sources:
http://archive.orr.noaa.gov/book_shelf/1476_OdorThreshold.pdf
http://blogs.scientificamerican.com/the-curious-wavefunction/chemists-and-bad-smells-and-sulfur-a-productive-pairing/
http://www.rrc.state.tx.us/media/8553/chap4-leakdetection-natgas.pdf
"The human sense of smell is very discerning and can detect mercaptans at a concentration of only 1 part per billion (ppb)."
Based on the above if my measurements and calculations are accurate, that would mean that I can detect it at about 5 parts per trillion. ???
Now, I've been in the house again several times, and though the stove hadn't been used in a long time, I could still smell gas. It wasn't nearly as strong as before, but still quite offensive. The smell hit me opening the outer door of the house already, before even opening the main door. The smell was strongest in the lower kitchen cabinets under the stove and under the sink - enough to make me gag. But again, no one else who visited the house smelled anything unusual, even in those same cabinets.
I strongly suggested the owner get the pipes and stove checked for a gas leak. But procrastination ensued.
Finally I procured a combustible gas detector, as by now I was now very curious whether it was really gas that I was smelling, and how much of it there was, that I could smell it so strongly, yet no one else could.
In the cabinet under the stove, the detector showed levels of up to 6000ppm of natural gas, or 11% of the LEL (lower explosive limit). Inside the rest of the house, where the odor was still fairly strong to me, it showed levels of about 50ppm.
The owner finally did get it checked out. The gas company rep confirmed a "small" leak (not with his nose; with his own meter), and shut off the gas. There was a shut-off valve right in the cabinet which I hadn't even noticed due to being so repulsed by the smell.
The house was aired out. Now, with the main gas smell gone, I noticed that the house smelled rather musty - something I hadn't been able to tell before, due to the stronger smell of gas. But I still kept noticing some whiffs of gas too. So I checked the cabinets again, and found another leak, this time by the pipes under the oven. There, the detector showed levels of only up to 300ppm, but it was still strong enough to make me gag.
If the gas rep considered levels of 6000ppm a small leak, I suppose that levels of 300ppm (~0.5% LEL) would be considered inconsequential. And the 50ppm or so (~0.1%), which I could also easily detect, would be considered even more inconsequential.
Anyway, I found a pamphlet which says:
"Both propane and natural gas are colorless and odorless. For safety reasons, an odorant (Mercapatan) is added so that any leak can be detected by smell. The common detection threshold for smelling the gases is around 20% of the lower explosion limit (LEL)."
So apparently most people can smell it at 20%, but I can smell it at 0.1%. I'm really curious if this is genetic, and how many other people there are who are similarly sensitive to the smell of mercapatan.
Also, propane smells different to me than natural gas does. I wonder if they are they truly odorless to everyone, and is it only the additives that I smell? But it may be that the smell I associate with propane is the smell of the smoke when it is burned, more than the gas itself.
I also wonder now, how many smells which are so strongly offensive to me, other people don't even smell at all. (As opposed to them smelling it but not being bothered by it, as I had generally assumed.) I've been thinking about this a lot lately in light of the above, and might do another post on it.
Oh, another note. According to several sources:
http://archive.orr.noaa.gov/book_shelf/1476_OdorThreshold.pdf
http://blogs.scientificamerican.com/the-curious-wavefunction/chemists-and-bad-smells-and-sulfur-a-productive-pairing/
http://www.rrc.state.tx.us/media/8553/chap4-leakdetection-natgas.pdf
"The human sense of smell is very discerning and can detect mercaptans at a concentration of only 1 part per billion (ppb)."
Based on the above if my measurements and calculations are accurate, that would mean that I can detect it at about 5 parts per trillion. ???